Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. There are several different types of memory used in modern electronics, one common type is RAM (random-access memory). RAM is characteristically found in use as main memory in a computer environment. RAM functions as a read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to read-only memory (ROM), which permits you only to read data. Most RAM, such as dynamic RAM (DRAM), static RAM (SRAM) and synchronous DRAM (SDRAM), is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
Computers almost always contain a small amount of ROM that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to. An EEPROM (electrically erasable programmable read-only memory) is a special type non-volatile ROM that can be erased by exposing it to an electrical charge. EEPROM comprise a memory array which includes a large number of memory cells having electrically isolated gates. Data is stored in the memory cells in the form of charge on the floating gates or floating nodes associated with the gates. Each of the cells within an EEPROM memory array can be electrically programmed in a random basis by charging the floating node. The charge can also be randomly removed from the floating node by an erase operation. Charge is transported to or removed from the individual floating nodes by specialized programming and erase operations, respectively.
Yet another type of non-volatile memory is a Flash memory. A Flash memory is a type of EEPROM that is typically erased and reprogrammed in blocks instead of a single bit or one byte (8 or 9 bits) at a time. A typical Flash memory comprises a memory array, which includes a large number of memory cells. Each of the memory cells includes a floating gate field-effect transistor (FET) capable of holding a charge. The data in a cell is determined by the presence or absence of the charge in the floating gate/charge trapping layer. The cells are usually grouped into sections called “erase blocks.” Each of the cells within an erase block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation, wherein all floating gate memory cells in the erase block are erased in a single operation.
The memory cells of both an EEPROM memory array and a Flash memory array are typically arranged into either a “NOR” architecture (each cell directly coupled to a bit line) or a “NAND” architecture (cells coupled into “strings” of cells, such that each cell is coupled indirectly to a bit line and requires activating the other cells of the string for access).
Floating gate memory cells are typically programmed by injecting electrons to the floating gate by channel hot carrier injection (CHE), placing the cell in a high threshold voltage state, and can be erased by hot hole injection from the substrate. Alternatively, floating gate memory cells can be programmed and erased by electron tunneling from and to the substrate by Fowler-Nordheim tunneling to put the cell in a programmed or erased threshold state. Both mechanisms require significant amounts of power and the generation of high positive and negative voltages in the memory device which can place high fields across the gate insulation layers with resulting adverse effects in device characteristics and reliability.
A problem with CHE, hot hole injection and Fowler-Nordheim tunneling is that the high energy required for their operation damages the device materials, reducing memory cell lifetimes and endurance. They can also consume large amounts of power, which is a problem in portable devices. In addition, the high voltages and fields limit the device feature scalability of the array and its support circuitry and significantly slow the write, read and erase speed of the resulting device. In particular, with Flash memory device types, CHE electron injection can generate interface states, degrade device transconductance, and enhance back-tunneling that affects charge retention and read-disturb. Fowler-Nordheim tunneling and hot hole injection can generate fixed charge centers in the tunneling insulators and shallow traps and defects in the trapping layer, thus breaking stable bonds and eventually degrading the insulator/dielectric properties of the device (limiting device endurance to a typical lifetime of less than 106 program/erase cycles). Such high power, high voltage, slow access speed, limited endurance and scaling difficulties are a typical characteristic of most commonly utilized non-volatile memory devices.
An ideal or universal memory would combine the high speed, low power and effectively infinite (1012 to 1015 program/erase cycles) write and erase endurance of RAM with the non-volatile long term data retention of a non-volatile memory. Such a memory device could be utilized by system designers to supplement or even entirely replace both RAM and ROM/Flash/non-volatile memory in computer systems and in portable devices.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for methods and apparatus for a non-volatile memory cell that allows for a non-volatile memory with high speed write/read/erase access, low voltage program and erase, low power usage, device feature scalability and effectively infinite endurance.